[cfe-dev] Diagnostic IDs, parsing speed and how to generate lookup tables

[Chris Lattner]

On Dec 12, 2012, at 2:45 PM, John McCall <rjmccall at apple.com> wrote:

>> That led to the question posed by Dmitri on irc why there are start ranges in the first place - we could instead tablegen all diagnostics at once and let the tablegen take care of generating the appropriate enum values for the start of certain ranges.
>> 
>> So if anybody has better ideas for how to solve the lookup problem, or knows why the diagnostic ids have fixed start values, I'd be very interested to learn more about it :)
> 
> The purpose of the fixed starting values is just compile-time efficiency: we'd like to be able to add/remove diagnostics (generally in one part of clang) without requiring a full recompile.  As long as your scheme still allows this in most cases — maybe tblgen gets re-run for all diagnostics whenever any of them change, but tblgen rounds up to the next multiple of 50 between ranges so that adding a diagnostic in one range doesn't usually force a full recompile — I think we're fine.

Right.  Basically move the magic constants in DiagnosticIDs.h:

   enum {
      DIAG_START_DRIVER        =                               300,
      DIAG_START_FRONTEND      = DIAG_START_DRIVER          +  100,
      DIAG_START_SERIALIZATION = DIAG_START_FRONTEND        +  100,
      DIAG_START_LEX           = DIAG_START_SERIALIZATION   +  120,
      DIAG_START_PARSE         = DIAG_START_LEX             +  300,
      DIAG_START_AST           = DIAG_START_PARSE           +  400,
      DIAG_START_COMMENT       = DIAG_START_AST             +  100,
      DIAG_START_SEMA          = DIAG_START_COMMENT         +  100,
      DIAG_START_ANALYSIS      = DIAG_START_SEMA            + 3000,
      DIAG_UPPER_LIMIT         = DIAG_START_ANALYSIS        +  100
    };

from the header file into tblgen.

-Chris
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